Safety is one of the most important factors in the design and use of the radiofrequency (RF) components of magnetic resonance (MR) imaging scanners. Application of high RF power levels for extended times causes a risk of patient injury due to increase in temperature of the targeted tissue region. One parameter used to characterize MR imaging safety for RF coils is the specific absorption rate (SAR). The SAR is a measure of the power absorbed per mass of tissue and is usually expressed in units of watts per kilogram (W/kg).
One method to determine the SIR involves calculating the overall average, or global, SAR. The global SAR is a measure of the average power absorbed per unit mass of tissue that is delivered to the entire mass for the body part under investigation in the patient.
Determining the SIR has become increasingly important in view of the increasing field strengths that are being utilized for MR imaging. These increasing field strengths improve image signal-to-noise ratio and contrast but result in higher SAR values and higher spatial variation of local SAR.
Some MR imaging scanners utilize parallel transmit technology, in which multiple channels are controlled independently. While parallel transmit technology helps to mitigate some of the aforementioned problems, an unconstrained combination of the power output to multiple channels can cause strong local heating effects due to possible constructive interference. As will be appreciated, this leads to high local SAR.
Various methods have been proposed to monitor the spatially varying SAR pattern to promote MR imaging safety. One method requires the measurement of spatial temperature variation within the body of a patient. The downside of this method is that temperature and SIR patterns are not identical to each other, mainly due to the effects of heat conduction and blood perfusion, which act to smooth and otherwise alter the SAR patterns. Additionally, MR imaging thermometry has a temperature resolution of approximately 1 degree Celcius. Given that the upper limit value of temperature rise in terms of safety is set to 1 degree Celcius as specified in IEC standards (IEC 60601-2-33: Particular requirements for the safety of magnetic resonance equipment for medical diagnosis), the use of MR imaging thermometry to monitor safety in MR imaging is not practical.
U.S. Patent Application Publication No 2010/0076298 to Gross discloses a method to determine the spatial distribution of the specific absorption rate in tissue that represents a measure of the absorption of electromagnetic fields emitted by means of a radiation generating element, wherein at least one item of measurement information acquired by a thermoacoustic computed tomography device is used to determine the specific absorption rate.
As be appreciated, improvements in estimating SAR are desired, it is therefore an object to provide a novel method and system for estimating the specific absorption rate of a tissue region prior to a MR imaging scan.